Ok, so I have a set up with a Power Acoustik amp that's pushing 8 years old (model no. lt980-2) and two alpine s subwoofers. Over the past six months or so I've had issues with a heartbeat like sound coming from my subwoofers, and the odd thing about it is that the heartbeat sound only occurs while driving over a bump, shutting a car door, closing the trunk, etc. Basically, the odd sound will only come out when the car is under some kind of contact.

Does anyone know what the deal is, or how I could fix it? When I had it installed all the wiring was fine, and it was grounded correctly. Thanks for any help.